Page 13 - Basic Principles of Textile Coloration

2 AN INTRODUCTION TO TEXTILES, DYES AND DYEING

exposure to a given agency or treatment. Most natural dyes also lacked
substantivity for fibres such as wool and cotton. Substantivity implies some
attraction of the dye for the fibre, so that the dye in the solution gradually
becomes depleted as it is absorbed by the fibres.

The poor substantivity and fastness properties of natural dyes often improved if
the fabric was first treated with a solution containing a salt of, for example, iron,
copper or tin. The conditions used favoured combination of the metal ions with
the particular fibre, or their precipitation inside it. These metal salts were called
mordants. When the pre-mordanted fabric was soaked in a bath of a suitable
natural dye, the dye penetrated into the fibres and reacted with the metal ions
present. This reaction decreased the water solubility of the dye so the colour was
less likely to bleed out on washing. The word ‘mordant’ originated from the French
verb mordre meaning ‘to bite’. In Chapter 13, we shall see that the idea of the dye
biting the mordant, to form a stable dye–metal complex, is a useful description. In
modern dyeing procedures, the dye reacts with the mordant in the fibre in a
separate process after dyeing, or the metal is incorporated into the dyestuff during
its manufacture.

A few natural dyes gave better quality dyeings of cotton or wool, but involved
long and difficult processes. For example, the colorant extracted from madder root,
from the plant Rubia tinctorium, dyed cotton pre-mordanted with aluminium and
calcium salts to give the famous Turkey Red. Using an iron mordant, the same
colorant gave a purplish-black.

Indigo, extracted from leaves of the plant Indigofera tinctoria, and Tyrian Purple
from Mediterranean sea snails of the genera Murex and Purpura, are water-
insoluble pigments called vat dyes. These do not require mordants. During the
time of the Roman Empire, wool cloth dyed with Tyrian Purple was so highly
prized that only the ruling class wore garments made with it. For dyeing with
Indigo, a water-soluble, reduced form of the dye was first obtained by extraction
and fermentation. The process became known as vatting, from the name of the
vessels used – hence the term ‘vat dye’. The soluble, reduced form of the dye is
called a leuco derivative. Leuco Indigo has substantivity for wool and cotton fibres.
After dyeing, air oxidation of the pale yellow leuco dye, absorbed in the fibres,
regenerates the dark blue, insoluble pigment trapped inside them. Because of this,
the fastness to washing is very good in comparison to most natural dyes. Scheme
1.1 outlines the essential steps in vat dyeing.